London Assurance, National Theatre

For the life of me I cannot understand why London Assurance is not performed more often. It’s a rollicking comedy, written in 1841 but which has a Restoration heart, with a cast list that includes a wideboy named Dazzle, a valet Cool, a servant Pert, a lawyer Meddle and - hold your sides - a horsey broad brandishing a whip named Lady Gay Spanker. Calm down, now.

Dion Boucicault’s comedy of manners (written when he was only 21) is a witty commentary on town versus country and many of its lines could have been written yesterday. Mostly, though, it’s a chance for some of our greatest thespians to overact like billy-oh to great effect as they romp through the Irish playwright’s witty evocation of a Regency London fop who travels to deepest Gloucestershire to marry his much younger bride in order to gain an inheritance.

Sir Harcourt Courtly, Bart (don’t forget the Bart) fancies himself as the exemplar of gentlemanliness. He is 57, but tries to pass for 40, even though he has a grown son and his ebony hair, complete with cowlicks, looks, shall we say, previously used. Simon Russell Beale is on terrific form as the self-deluded effete, all preening in front of mirrors and striking ridiculous poses to suggest how virile he still is. Sir Harcourt, having been deserted by his first wife, who eloped with his best friend - “I miss him” - is now betrothed to Grace (Michelle Terry), the 18-year-old niece of his friend Squire Max Harkaway (Mark Addy).

Sir Harcourt thinks his son, Charles (Paul Ready), is a dutiful student, but he is in fact a rake who is in massive debt and has gained an equally dissolute pal in the form of Richard Dazzle (Matt Cross). We meet them first at Sir Harcourt’s Belgravia Square townhouse, full of flunkeys led by a sneering, imperious Cool (the marvellous Nick Sampson, threatening to steal the show), and then the action moves to the more bucolic delights of Squire Harkaway’s house in Gloucestershire.

Charles, of course, falls instantly for Grace, a no-nonsense country girl who has resigned herself to a loveless (and one assumes, sexless) marriage to Sir Harcourt. But the old fellow in turn falls for Grace’s cousin, the magnificently named Lady Gay Spanker (the stupendous Fiona Shaw), who speaks entirely in hunting metaphors and has a truly memorable entrance, reliving every moment of her thrilling hunt across hedgerow, ditch and dale in hysterical mime.

Dupes, ruses and false identities ensue as various pairings couple and uncouple, but ultimately Grace and Charles get together. But theirs is not a soppy romantic love; their marriage is as much a financial as romantic contract and we see that not all happy partnerships are about passion - Lady Gay Spanker (I write that name in full again so that my companion last night can throw her head back in merriment once more) realises that her husband, Dolly (Richard Briers), may be dull, but he truly loves and cares for her, and that counts for more than physical passion. Meanwhile Sir Harcourt has an epiphany as he realises much of his existence is ridiculous.

Mark Thompson’s design is gorgeous (the costumes being particularly fine) and the production, by Nicholas Hytner, is an absolute joy; it often feels like the cast are having as good a time as the audience and could collapse into giggles at any moment. While the young lovers may fail to convince and there are odd moments of dead air on stage, as soon as Shaw, Russell Beale or Sampson reappear we are back on form. This is a real hoot.

• London Assurance is in rep at the Olivier, National Theatre until 2 June.